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Background: The Impact of Weight on Self-Perceptions Questionnaire (IW-SP) assesses an individual's self-perception related to his or her weight. The primary objective of this study was to provide evidence of the reliability, validity, and responsiveness of the IW-SP. Materials and Methods: Study participants were individuals with type 2 diabetes mellitus (T2DM) and obesity enrolled in clinical weight-loss programs in the United States. Data were obtained for clinical measures, IW-SP, and other patient-reported outcome measures. An intraclass correlation coefficient (ICC) and Cronbach's α were calculated for test-retest reliability and internal consistency, respectively. For validity, correlations and t tests were performed. For responsiveness, baseline and 6-month data for a subgroup of patients were analyzed using the paired t test and calculation of effect size (ES). Results: Reliability data for 106 study participants (mean age, 52 years; 69% female; 31% white; mean body mass index, 38 kg/m2) yielded an ICC of 0.85 and Cronbach's α values of >0.89. IW-SP scores were associated with obesity-related quality of life, mental health, and vitality ( r>0.50, P<0.001). In the subgroup ( n=40) used to estimate responsiveness, weight was significantly less at end point than at baseline (mean, baseline=231.9 vs. end point=222.0 pounds; P<0.001; ES=0.23), and IW-SP scores were significantly better than at baseline (mean, baseline=61.0 vs. end point=72.1 [on a scale of 0-100]; P=0.01; ES=0.34). Mean IW-SP change scores significantly discriminated between those achieving >5% body weight loss and those who achieved <5% (mean change, 23.6 vs. 5.7; P=0.03). Conclusions: The IW-SP has demonstrated reliability, validity, and responsiveness in individuals with T2DM and obesity, thereby making it a potentially valuable tool in the evaluation of weight-loss interventions targeted toward patients with T2DM. [ABSTRACT FROM AUTHOR]